I am entertaining the notion that with solenoids, pull-in current is a *lot* higher than hold-in current.
What would happen if you put, say, some 1 ohm, 100 watt power resistors ( BTW, that's a standard off-the-shelf resistor ) in series with the FSS, and bypass this resistor with the NO contacts of a relay, whose coil is powered from the starter relay coil ( or contacts ).
The idea being when you first start the engine, during the initial start, the FSS coil is run at Full Power. As soon as you release the starter, the FSS drops down to hold in current which has to come in via the resistors.
Note that resistor is designed to be mounted to a heat sink. In this case, the firewall should suffice. If you have some thermal transfer compound around, a dab wouldn't hurt. Start with maybe two in series.
Get several resistors, as you may want to series and parallel them to get other holding currents. By experiment. They come in lots of five. That oughta be sufficient for a bit of trial-and-error to get a robust hold-in with minimal hold current, then wire it in for good when you get it right. My guess is to start off with two one-ohm resistors in series.
The resistors will dissipate some of the heat that would have been dissipated in the FSS.
Note this does the same thing a "ballast resistor" did for the ignition coil primaries on the old cars of the kettering-ignition points era.
I have been lurking on this thread for a while, but haven't posted until I felt I had anything useful to add. I consider this relentless string of FSS failure intolerable. I see is it correlates with WMO. Maybe WMO has some heat transfer anomaly from diesel fuel and the FSS also has borderline thermal design, and you just happened to luck out to discover it.
I also entertain the fear that some accountant got into engineering and directed the factory to wind aluminum wire on the solenoid. I have seen accountant types screw up more legacy design with cost-cutting ideas that result in enormous disappointment for the end user.
(Incidentally, I have several USB cables I discovered that were made with aluminum wire when I tried to solder them. Aluminum simply wouldn't "wet" right. I seen them use iron wire too. And watch out for those cheap jumper cables! That CCA rating is not Cold Cranking Amperes.... Rather when referring to what looks like copper wire, it likely means "Copper Clad Aluminum"! )
A discovered a washing machine motor wound with aluminum wire when I tried to salvage some copper wire from it. The enamel was copper colored. The wire was aluminum.